1. Field of the Invention
This invention relates in general to networks using the ATM Adaptation Layer 2 protocol, and more particularly to a method for eliminating misconcatenation in AAL2.
2. Description of Related Art
ATM has been selected as a world standard for broadband ISDN in network communication systems. ATM systems have been implemented on a global basis and developed in a rapid growth. ATM technology is destined to play a major role in both public and private broadband networks. AAL2 is one of the four types of AAL (ATM Adaptive Layer) protocols which have been recommended by CCITT (now ITU-T), namely AAL1, AAL2, AAL3/4 and AAL5. In general, the layer services provided by AAL1 are constant bit rate (CBR) services which require information to be transferred between source and destination at a constant bit rate. AAL2 offers a transfer of information with a variable bit rate. In addition, timing information is transferred between source and destination. Since the source is generating a variable bit rate, it is possible that cells are not completely filled and that filling level varies from cell to cell. AAL3/4 is used for transfer of data which is sensitive to loss, but not sensitive to delay. The AAL3/4 protocol may be used for connection oriented as well as for connectionless data communication. AAL3/4 itself does not perform all functions required by a connectionless service, since functions like routing and network addressing are performed on the network layer. AAL5 is designed to offer a service with less overhead and better error detection below the common part of the convergence sublayer (CPCS).
The AAL2 signaling protocol describes methods by which a switched AAL2 connection can be established between two AAL2 end users across a network that consists of both ATM and AAL2 switches. The current activities in the Study Group 11-WP1/Q6 or ITU-T is focused on specifying the requirements for such a signaling protocol. The important function of AAL2 signaling protocol is to establish an AAL2 connection between two AAL2 end points on a concatenation of ATM Virtual Channel Connections (VCCs) that are either on demand (SVC) or semi-permanent (PVC). Some the basic requirements of AAL2 signaling protocol include the ability to establish an AAL2 connection between AAL2 end systems that support AESA formats, the ability to support hop-by-hop routing mechanism between AAL2 end systems, the ability to indicate any failures to corresponding management entity, and the ability to setup AAL2 connections with different QoS requirements.
In AAL2, packets (minicells) from many users are assembled into a single ATM cell and transmitted on the same ATM connection. In addition, packets are allowed to straddle across ATM cell boundary to maximize the bandwidth utilization. A single bit sequence number is defined in the CPS-PDU header to detect cell losses at the receiver. But, an even number of cell losses will render the sequence bit inadequate and resulting in misconcatenation of packets. The CPS-PDU header includes a length indicator, which can be used to identify the problem when the payload size does not match its value. If the size of the remainder of the payload of a packet matches the size of the partial packet encapsulated in an ATM cell that arrives after an even number of cell losses, then partial packets from two different users are misconcatenated and sent to the user whose CID was received in the previous partial packet. In traditional applications, misconcatenation is generally not a serious problem in other types of networks since all the information, albeit erroneous, belongs to the same user with the exception of a few bytes missing at random. Elimination of misconcatenation can be important in AAL2 since data belonging to one user is delivered to another user resulting in security violation and application malfunction.
Delay sensitive applications such as speech and interactive video can not afford re-transmission and checksum verification due to overhead and additional delays. In such cases, misconcatenation can cause application malfunction at the receiver. In addition, the misconcatenation of signaling and management information sent through AAL2 can lead to network breakdown.
To solve the problem of misconcatenation, a check sum (5 bit CRC) at the UUI field of the CPS-Packet header could be used. The User-to-User Information (UUI) field in the CPS-Packet is allocated for carrying CPS-Packet sequence number and other peer to peer control information. However, this field can be used for checksum when it is not used for any other purpose. Nevertheless, this scheme is complex in an AAL2 switching environment since packets are unpacked and packed at intermediate nodes more often than in a point-to-point AAL2 connection. The chance of a packet being split across cell boundary increases considerably in an AAL2 switching network. In order to safeguard the UUI information, intermediate nodes have to extract UUI and send it separately. which requires additional bookkeeping and signaling.
Another method to solve the problem is to use the CLP bit in the ATM cells to identify the problem. However, this solution has the limitation that it can only solve the problem of consecutive cell loss. By tracking the partial packets (4 cells) during the cell assembly process at the transmitter, the critical cell (cell 3) can be tagged to guarantee that the network will deliver the critical cell to the receiver. However, this method also requires additional variables and modification in the current CLP handling procedures in the ATM switch.
It can be seen that there is a need for a method for eliminating misconcatenation in AAL2 that is simple and which does not require additional variables or modification of current CLP handling procedures.